This invention relates to a power conversion system for converting electric power from alternating current to the form of direct current and vice versa.
There have been proposed numerous types of converters for converting alternating current (AC) power to direct current (DC) power as well as numerous types of inverters for converting DC power to AC power. Representative examples of the converter involve rectifier devices.
In some electric apparatus, for example, uninterruptible power supplies or standby power supplies, it is required to perform the operation of converting AC power to DC power as well as the operation of converting DC power to AC power. Conventional electric apparatus having both the function of converting AC power to DC power and the function of converting DC power to AC power have generally included both a converter for converting AC power to DC power and an inverter for converting DC power to AC power. Converters are generally equal to or higher than inverters in cost. Thus the use of two conversion devices (a converter and an inverter) has resulted in electrical which is expensive and large-sized.
In order to avoid these objections, it has been already proposed to selectively use a single conversion device as a converter and an inverter by changing the electrical connection of the device between the AC-to-DC and DC-to-AC conversion modes of operation. This method is effective for electric apparatus having the time period during which they are operated in the AC-to-DC conversion mode, not overlapping the time period during which they are operated in the DC-to-AC conversion mode. The uninterruptible and standby power supplies as above described are just such an electric apparatus. In such electric apparatus the selective use of a single conversion device as a converter and an inverter can reduce the size thereof while nearly halving the cost. However, since the electrical power connection should be changed between the AC-to-DC and DC-to-AC conversion mode of operation, time has been required until the AC-to-DC conversion mode is switched to the DC-to-AC conversion mode of operation and vice versa. This has led to power breakdown in uninterruptible power sources during that switching time. In addition, the switching has required an additional control circuitry.
Accordingly it is an object of the present invention to provide a new and improved power conversion system operative in either of the AC-to-DC and DC-to-AC conversion modes with a single conversion device and without changing the electrical connection of the device.